1. Field of the Invention
This invention relates to a propelled pyrotechnic decoy flare, and in particular to a decoy flare that can be aircraft-launched to lure incoming missiles with advanced infra-red seeker systems away from the aircraft's exhaust.
2. Discussion of Prior Art
Known decoy flare compositions comprise magnesium and polytetrafluoroethylene (hereafter PTFE) mixtures pressed to form pellets. A pellet is then launched from an aircraft when an incoming missile is detected. The pellet is ignited on launch and burns to produce an infra-red source more intense than the aircraft exhaust. If the incoming missile has an infra-red seeker system then the missile can be lured away from the aircraft exhaust to the more intensely burning pellet which falls quickly away from the aircraft.
Several types of advanced infra-red seeker systems are in use in anti-aircraft missiles which are designed to recognise the typical characteristics of a decoy flare and ignore it. One such infra-red seeker system is sensitive to the sudden increase in infra-red output intensity in the area of the aircraft exhaust produced when a decoy flare is ignited. When the infra-red seeker system detects a sudden increase in infra-red output intensity it activates countermeasure circuitry for a short time which causes the seeker system to memorise and continue to follow its original trajectory (ie the trajectory calculated by the seeker system from the position and velocity of the aircraft exhaust to lead to impact with the aircraft) ignoring all infra-red sources. The said short time, typically around 0.2 second is chosen so that when the countermeasure circuitry is deactivated a conventional decoy flare will be outside the seeker system's field of view and so the only infra-red source the seeker system recognises is the aircraft exhaust. Thus the missile will continue to track the aircraft exhaust. Another such infra-red seeker system is sensitive to the rate at which the decoy flare separates from the aircraft. When a conventional decoy flare is launched from the aircraft it decelerates rapidly and falls under gravity and so separates rapidly from the aircraft. When the infra-red seeker system detects a second infra-red source it measures the rate of separation of the two sources and memorises and continues to follow its original trajectory. If the rate of separation is above a predetermined level the seeker system will ignore the second source and continue to trace the aircraft exhaust. It takes the seeker system only about 0.2 second to measure the rate of separation. Other advanced infra-red seeker systems use a combination of the two systems described above. Clearly if an infra-red decoy flare is to be effective it must be able to overcome all types of advanced seeker systems.
A known method of overcoming a range of advanced seeker systems is to launch and ignite sequentially a plurality of decoy flares from the aircraft and for the aircraft to simultaneously manoeuvre away from the trajectory of the missile. The principle behind this being that while the seeker system is sequentially detecting and analysing each of the plurality of decoy flares it continues to follow its original memorised trajectory so that by the time the last decoy flare has burnt out the aircraft will have manoeuvred so that the aircraft exhaust tubes face away from the missile and the seeker system no longer recognises the aircraft as its target. A disadvantage of this method is that the plane has to carry a large number of decoy flares which take up a large amount of space in the aircraft. A further disadvantage is that the aircraft has to manoeuvre away from the trajectory of the missile and so is not able to take the most direct route out of a hostile region.